Before the change, I was unable to compile anything. Note the highlighted text below.

# /usr/vac/bin/xlc cgc.c

The license for the Evaluation version of IBM XL C/C++ for AIX V11.1 compiler product has expired. Please send an email to compiler@ca.ibm.com for information on purchasing the product. The evaluation license can be extended to 74 days in total by either a) setting an environment variable XLC_EXTEND_EVAL=yes; or, b) specifying a compiler command line option -qxflag=extend_eval. The extended evaluation license will expire on Sat Jun 18 10:26:33 2011. Use of the Program continues to be subject to the terms and conditions of the International License Agreement for Evaluation of Programs, including the accompanying License Information document (the "Agreement"). A copy of the Agreement can be found in the "LicAgree.pdf" and "LicInfo.pdf" files residing in the root directory of the installation media. If you do not agree to the terms and conditions of the Agreement, you may not use or access the Program.

With the environment variable in place, I was able to compile again.

# /usr/vac/bin/xlc cgc.c

# ./a.out

Hello World!

Of course I could also export the environment variable as required instead e.g.

# export XLC_EXTEND_EVAL=yes

# /usr/vac/bin/xlc cgc.c

# ./a.out

Hello World!

If you do place this environment variable in /etc/profile, you may need to restart any processes on the system that need to call the compiler.